The present invention relates to a microlithographic process using a photosensitive composition, a micromachining technique for preparing a semiconductor device, or the like, and a process for preparing the semiconductor device, or the like, including the microlithographic process. More particularly, the present invention relates to a method for forming a negative pattern suitable for a photolithographic process using a far ultraviolet radiation having a wavelength of not more than 220 nm, such as an ArF excimer laser, which is a radiation source for wavelengths shorter than those in currently used UV sources including high pressure mercury lamps and KrF excimer lasers.
A photolithographic technique for fabricating micro-order or submicron-order fine patterns within an electronic device, such as a semiconductor, has been at the core of micromachining techniques for mass production. A demand for an increase in integration and an increase in density of semiconductor devices in recent years has led to many improvements in micromachining techniques. In particular, as the minimum possible machining dimension approaches the exposure wavelength, photolithographic techniques using light sources for shorter wavelengths ranging from g-line (436 nm) and i-line (365 nm) in a high pressure mercury lamp to a KrF excimer laser (248 nm), have been developed. A change in exposure wavelength has led to the development of photoresists made from materials suitable for the wavelength being used. In any of the photoresists suitable for the above wavelengths, aqueous alkali development utilizing the solubility of resins having a phenol structure or polymeric materials in an aqueous alkali has been adopted on a commercial scale, although they are different from each other in photoactive compound and photoactivating mechanism. These resins and polymeric materials necessarily contain many aromatic rings which are also chemical structural elements for enhancing the etching resistance in the step of dry etching after the formation of the resist pattern.
Negative resists prepared from resins having the above-mentioned phenol structure are classified into ones of a crosslinking type as disclosed in Japanese Patent Laid-Open No. 164045/1987 and ones of a dissolution inhibition type as disclosed in Japanese Patent Laid-Open No. 165359/1992. In any type, submicron-order fine patterns can be formed without causing swelling.
Photolithography using an ArF excimer laser (193 nm) as a light source has become more and more commonplace for photolithography which can provide a minimum possible machining dimension of less than 0.25 xcexcm. This wavelength corresponds to the absorption maximum of an aromatic ring. Therefore, in the photoresist material composed mainly of a phenol structure, which has been used on a commercial scale, the formation of a latent image in response to the exposure is limited to a portion which is very near the surface of the photoresist film, making it difficult to form a fine resist pattern by aqueous alkali development.
On the other hand, various resist materials have been proposed which have high transmittances in the above-mentioned wavelength region while possessing high dry-etching resistance. Utilization of an adamantane structure (Japanese Patent Laid-Open Nos. 39665/1992 and 265212/1993) and utilization of a norbornane structure (Japanese Patent Laid-Open Nos. 80515/1993 and 257284/1993) have been proposed as chemical structures which are transparent in the far ultraviolet region including the wavelength 193 nm of the ArF excimer laser and, instead of the aromatic ring, can impart dry-etching resistance to the resist material. Japanese Patent Laid-Open Nos. 28237/1995 and 259626/1996 disclose that, in addition to these structures, general alicyclic structures, such as a tricyclodecanyl group, are also useful.
Regarding resist materials which have been rendered developable with an aqueous alkali by using a polymer having a chemical structure transparent in the far ultraviolet region including the wavelength 193 nm of the ArF excimer laser and, utilization of the carboxylic acid structure of acrylic acid or methacrylic acid, has been attempted in Japanese Patent Laid-Open Nos. 39665/1992, 184345/1992, 226461/1992, and 80515/1993. In these publications, the aqueous alkali solubility of a portion dissolved in a developing solution in the aqueous alkali development depends upon the carboxylic acid structure of acrylic acid or methacrylic acid. Furthermore, Japanese Patent Laid-Open No. 8-259626 discloses a polymeric compound in which a carboxylic acid group has been provided in an alicyclic structure introduced into the side chain of a methacrylic acid ester.
The phenol structure which has so far been used as an alkali-soluble group has a pKa of 10.0 (phenol), whereas the above carboxylic acid structure has a pKa of as low as 4.8 (acetic acid), indicating that its acidity is high. Therefore, when they are used as the alkali-soluble group of the base resin, the results is that with the same molar fraction the resin having a carboxylic acid structure has a higher dissolution rate in an aqueous alkali and in addition can be dissolved in an alkali developing solution having such a low concentration that it will not dissolve the resin having a phenol structure.
Use of the resin having a carboxylic acid structure in combination with a cross-linking agent as described in Japanese Patent Laid-Open No. 164045/1987 is disadvantageous because it causes the carboxylic acid having high acidity to remain in the crosslinked portion and an alkali developing solution infiltrates into this portion and swells, which makes it impossible to form a fine pattern. On the other hand, when use is made of a material described in Japanese Patent-Laid Open No. 165359/1992, wherein the acid formed upon exposure produces a compound having a dissolution inhibitory activity, no contrast in dissolution is created if the resin has a carboxylic acid structure, thereby making it impossible to provide a negative resist disadvantageously.
The fact that an acid-catalyzed reaction of a carboxylic acid with an alcohol in a solution results in conversion of the carboxylic acid to a carboxylic acid ester is generally known in the art, and this reaction is one useful method for synthesizing a carboxylic acid ester. Since, however, this reaction is an equilibrium reaction, it is necessary for the reaction to proceed toward the ester side. Thus, alcohol should be used in large excess and, in addition, water as a by-product should be discharged outside the system. When the application of this reaction to a radiation-sensitive composition is contemplated, the alcohol cannot be added in large excess while maintaining the solubility of the coating in an alkali. Further, in this case, it is difficult to discharge water produced in the reaction outside the system. For this reason, in general, the esterification proceeds to an extent of only several percent. A reduction in the amount of the carboxylic acid to this extent is unsatisfactory for creating contrast of dissolution high enough to form a pattern.
A first object of the present invention is to provide a method for negative pattern formation which enables development using an aqueous alkali developing solution without causing swelling of a fine pattern, utilizes esterification of a resin having a carboxylic acid structure, and provides a fine pattern with excellent resolution.
A second object of the present invention is to provide a process for preparing a semiconductor device using the method for pattern formation according to the first object of the present invention.
The first object of the present invention can be attained by a method for pattern formation, including the following steps: forming a coating having a photosensitive composition containing at least a carboxylic acid structure on a predetermined substrate; irradiating the coating with an actinic radiation in a predetermined pattern form to form a desired latent image pattern in the coating; heating the exposed coating to allow the reaction to proceed; and then developing the heated coating with an aqueous alkali developing solution to form a desired pattern in the coating, wherein the carboxylic acid structure in the areas irradiated with the actinic radiation is partially or entirely converted to a xcex3-lactone structure or a xcex4-lactone structure which is a carboxylic acid ester structure. xcex3-lactone structures and xcex4-lactone structures are easier to form and five and six member links yield better results.
Since the xcex3-lactone or xcex4-lactone structure, is formed when the alcohol group to be esterified with the carboxylic acid group is one located at the xcex3-position or the xcex4-position of the carboxylic acid group in the molecule, the esterification by the acid-catalyzed reaction occurs more easily than that in the conventional method. Also, the ester thus prepared is not hydrolyzed by an aqueous tetraalkylammonium hydroxide solution commonly used in the art and remains stable during development.
The structure for creating the xcex3-lactone or xcex4-lactone structure is preferably a xcex3-hydroxy carboxylic acid structure or a xcex4-hydroxy carboxylic acid structure. In this structure, intramolecular esterification by the acid-catalyzed reaction results in the formation of a five or six-membered ring. Thus, the esterification can easily occur.
The carboxylic acid structure used in the method for pattern formation according to the present invention is preferably a chemical structure represented by the following chemical formulas (1) or (2); 
In these formulas, R1, R2, R3, R4, R5, R6, R7, and R8 each represent a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, provided that a plurality of alkyl groups as the substituent may be linked with each other to from a cyclic alkyl group.
Furthermore, preferably, at least the carboxylic acid structure represented by chemical formulas (1) or (2) is contained in the polymeric compound constituting the photosensitive composition.
Specifically, resins each containing at least a repeating structure selected from among the following general chemical formulas (5) to (12) are preferred because they have the above structures and, further, have small absorption in the far ultraviolet region and dry-etching resistance. 
In above formulas (5)-(12), n is an integer
The resin having at least the repeating structure represented by the formulas (5) or (6) may be prepared by the free-radical copolymerization of a compound containing an alicyclic hydrocarbon-structure, such as 5-methylenebicyclo[2.2.1]hept-2-ene, with maleic anhydride, reducing the maleic anhydride moiety to form a lactone, and conducting hydrolysis to modify the lactone.
The resin having at least a repeating structure represented by formulas (7) or (8) may be prepared by the free-radical copolymerization of a compound containing an alicyclic hydrocarbon structure, such as cycloocta-1,5-diene, with maleic anhydride, reducing the maleic anhydride moiety to form a lactone, and conducting hydrolysis for modification.
The resin having at least a repeating structure represented by formulas (9) or (10) may be prepared by the free-radical copolymerization of a compound containing an alicyclic hydrocarbon structure, such as 5-ethylenebicyclo[2.2.1]hept-2-ene, with maleic anhydride, reducing the maleic anhydride moiety to form a lactone, and conducting hydrolysis for modification.
The resin having at least a repeating structure represented by formulas (11) or (12) may be prepared by the free-radical copolymerization of a compound containing an alicyclic hydrocarbon structure, such as 5-vinylbicyclo[2.2.]hept-2-ene, with maleic anhydride, reducing the maleic anhydride moiety to form a lactone, and conducting hydrolysis to modify the lactone.
The weight average molecular weight of these resins is preferably 1,000 to 300,000. Among the above polymeric compounds, those having repeating units of an alicyclic structure having on its side chain the carboxylic acid structure represented by chemical formulas (1) or (2) are more advantageous over those containing directly in the backbone thereof the carboxylic acid structure represented by chemical formulas (1) or (2) because the lactonization in the acid-catalyzed reaction occurs more easily and higher sensitivity is achieved.
Specifically, a resin having repeating units of an alicyclic structure having on its side chain the carboxylic acid structure represented by chemical formulas (1) or (2), such as an alicyclic structure represented by the chemical formulas (5), (6), (9), (10), (11), or (12) is more advantageous than a resin containing directly in its backbone the carboxylic acid structure represented by the formula (7) or (8) because the pattern formation can be carried out with higher sensitivity.
When the carboxylic acid structure represented by chemical formulas (1) or (2) is contained directly in the backbone, the carboxylic acid moiety and the hydroxyl group moiety in chemical formulas (1) or (2), in some cases, become sterically distant from each other, making it difficult to lactonize the carboxylic acid structure. On the other hand, when the carboxylic acid structure represented by chemical formulas (1) or (2) is contained in the side chain, the carboxylic acid moiety and the hydroxyl group moiety are less likely to become sterically distant from each other. As a result, in such a case, the carboxylic acid structure can be easily lactonized, thereby possibly contributing to pattern formation of higher sensitivity.
A resin having the above-mentioned structures is used in combination with 0.1 to 30 parts by weight, based on the resin, of a compound capable of producing an acid upon irradiation with an actinic radiation to prepare a pattern forming material. The compounds capable of producing an acid upon irradiation with an actinic radiation include onium salts, such as triphenylsulfonium triflate, sulfonyloxyimides, such as trifluoromethanesulfonyloxynaphthylimide, and sulfonic acid esters. However, some other compound may be used if they can produce an acid upon irradiation with an actinic radiation, such as by an ArF excimer laser, for example.
The carboxylic acid structure used in the method for pattern formation according to the present invention may be also a chemical structure represented by the following chemical formulas (3) or (4): 
In these formulas, R1, R2, R3, R4, R5, R6, R7, and R8 each represent a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, provided that a plurality of alkyl groups as the substituent may be linked with each other to from a cyclic alkyl group; and X represents an acetal or ketal, such as a 1-ethoxyethyl or tetrahydropyranyl group. In this case, since the hydroxyl group is protected by X, the thermal stability is improved.
Further, preferably, at least the carboxylic acid structure represented by chemical formulas (3) or (4) is contained in the polymeric compound constituting the photosensitive composition.
Among the above-mentioned polymeric compounds, those having repeating units of an alicyclic structure having on its side chain the carboxylic acid structure represented by the chemical formulas (3) or (4) are more advantageous than those containing directly in the backbone the carboxylic acid structure represented by chemical formulas (3) or (4) because the lactonization in the acid-catalyzed reaction can occur more easily, which possibly contributes to higher sensitivity.
When the carboxylic acid structure represented by chemical formulas (3) or (4) is contained directly in the backbone, the carboxylic acid moiety and the protected hydroxyl group in chemical formulas (3) or (4), in some cases, become sterically distant from each other, thereby making it difficult to lactonize the carboxylic acid structure. On the other hand, when the carboxylic acid structure represented by chemical formulas (3) or (4) is contained in the side chain, the carboxylic acid moiety and the protected hydroxyl group moiety are less likely to become sterically distant from each other. As a result, in such a case, the carboxylic acid structure can be easily lactonized, thereby possibly contributing to pattern formation of higher sensitivity.
The photosensitive composition containing a carboxylic acid structure used in the method for pattern formation according to the present invention may further contain an alicyclic structure for enhancing the dry-etching resistance. The alicyclic structures include adamantyl, norbornane, tricyclodecane, and androstane structures. These structures are transparent in the far ultraviolet region and possesses dry-etching resistance.
The types of actinic radiation used in the present invention include a far ultraviolet light having a wavelength of not more than 250 nm and a vacuum ultraviolet light, such as ArF excimer laser. Additionally, electron beam, EUV, and X-radiation may also be used. The aqueous alkali developing solution used in the present invention is preferably an aqueous solution of a tetraalkylammonium hydroxide having 1 to 5 carbon atoms.
The second object of the present invention can be attained according to a process for preparing a semiconductor device, including the following steps: forming a resist pattern on a semiconductor substrate by any one of the methods for pattern formation described above; and, based on the resist pattern, either etching the semiconductor substrate or implanting ions into the semiconductor substrate. The etching methods usable in the process for preparing a semiconductor device include dry etching methods, such as plasma etching, reactive ion etching, and reactive ion beam etching, and wet etching.
Substrates treatable in the process for preparing a semiconductor device according to the present invention include silicon dioxide films formed by CVD or thermal oxidation, oxide films such as coatable glass films, and nitride films, such as silicon nitride film. Additional examples of substrates usable in the process of the present invention include films of various metals, such as aluminum and alloys thereof and tungsten, and polycrystalline silicon.
According to the present invention, utilization of a highly reactive lactonization method among methods of esterification of carboxylic acids enables the carboxylic acid to be efficiently converted to a carboxylic acid ester through a reaction catalyzed by an acid produced upon exposure. Since this reaction is an intramolecular esterification, intermolecular cross-linking and the like do not occur, thereby permitting the content of the carboxylic acid in the exposed areas to be simply rendered different from that in the unexposed areas. When the carboxylic acid and the alcohol are separate molecules, only several % of the carboxylic acid in the coating can be esterified by the acid-catalyzed reaction under practically applicable conditions. By contrast, in xcex3-lactonization or xcex4-lactonization used in the method for pattern formation according to the present invention, not less than 30% of the carboxylic acid is esterified in the exposed areas. Preferably, at least 50% is esterified and, even more preferably, at least 70% is esterified. This results in significantly changed dissolution rate in the exposed areas and, in addition, no cross-linking reaction occurs. As a result, the prior art problem of swelling can be avoided, and a fine pattern can be formed.
These and other objects, features and advantages of the present invention will be readily apparent in view of the following detailed description of the preferred embodiments in conjunction with the drawings.